The present invention relates to the field of coal-fired furnaces and, more particularly, to pulverized coal-fired furnaces designed as direct-fired systems. More specifically, the present invention is directed to the cold start light-off of a pulverized coal-fired furnace supplied with coal from one or more coal drying and pulverizing mills.
In order to avoid the high cost of oil and gas, electric utilities have increasingly chosen coal as the fuel to fire the furnaces of their steam generating boilers. However, even in coal-fired furnaces, substantial quantities of oil and gas are often used in starting and warming up the furnace. In a typical coal-fired unit, the coal must be pulverized in a pulverizer, often termed a mill, and fired by heated air before it can be burned in the furnace. The heated air used to dry the coal is supplied by a force draft fan that forces the air through a preheater wherein the air is passed and heat exchange with hot combustion products leaving the furnace.
Therefore, it is necessary that the furnace be already operating in order to dry the coal in the load-carrying mill for the coal to be burned in the furnace. Accordingly, in a typical coal-fired furnace, a relatively large oil burner is started by an igniter and operated for a fairly long period of time to warm up the furnace walls and the heat exchange surfaces of the air preheater. Once the furnace has been brought up to temperature, the load carrying mills can be brought on line and pulverized coal supplied to the furnace and ignited by oil or gas igniters associated with the coal burner.
Due to the expense of oil and gas, even when used simply as a warm-up fuel, it has generaly been considered necessary to burn such fuels in order to warm up the furnace since the combustion of pulverized coal with its moisture content is very difficult unless the coal is suitably dried prior to combustion. One proposal to reduce the consumption of oil or gas as a warm-up fuel is discussed in U.S. Pat. No. 4,090,455. As disclosed therein, a direct-fired air heater using oil as a fuel is used at start-up to generate hot air for the load carrying mills. The hot air from the auxiliary direct-fired air heater is mixed with ambient air from the main air preheater and passed to the load carrying mills as the sole source of hot air for drying the coal being pulverized in the mill during startup. The pulverized coal from the load carrying mills is then passed to the furnace and ignited by conventional oil or gas pilot igniters to produce a warm-up flame therein.
Another proposal for minimizing the use of auxiliary fuel such as oil or gas by warming the furnace up on pulverized coal, is discussed in U.S. Pat. No. 4,173,189. An ignition, warm-up and low load stabilization system are disclosed therein wherein a separate pulverizing mill, dedicated for start-up, is used to produce dried pulverized coal for start-up. Hot air from an independent source is supplied to the start-up pulverizer to dry the coal therein. The pulverized coal is exhausted from the pulverizer in the hot air and passed to a separation device wherein the pulverized coal is removed from the air. This separated pulverized coal is then passed in a dense phase stream to coal-fired igniter burners which are used to warm the furnace up.
A further system designed to minimize the use of auxiliary fuel by warming the furnace up on pulverized coal is disclosed in U.S. Pat. No. 4,241,673. As proposed therein, coal is pulverized and dried when the furnace is in normal operation and stored for subsequent use in a storage bin. When it is necessary to warm the furnace up, the pulverized coal is fed to the furnace from the storage bin, typically in a dense phase stream and ignited in the furnace by auxiliary ignition means. Although oil or gas igniters may be used for the auxiliary ignition means, it is disclosed therein that an electric spark ignition means is preferred so as to eliminate oil or gas as an auxiliary fuel and the start-up and warm-up of the furnace.
Accordingly, it is an object of the present invention to provide for the cold start light-off of a pulverized coal furnace utilizing pulverized coal from the load-carrying mills as the premdominant source of fuel for the start-up and warm-up of the furnace while minmizing or eliminating the use of auxiliary fuel such as oil or gas.